1. Field of the Invention
The present invention relates to a circuit for detecting a capacitance change in a variable capacitor, the capacitance of which changes according to a physical amount applied thereto, and more particularly to a circuit for detecting an amount of acceleration based on a capacitance change.
2. Description of Related Art
A device for detecting a physical amount such as acceleration or pressure based on capacitance changes in a capacitor is known hitherto. For example, JP-A-8-145717 discloses a circuit for detecting a capacitance difference between a sensor capacitor having a variable capacitance and a reference capacitor having a fixed capacitance. The detecting circuit includes a switched capacitor circuit composed of an operational amplifier, a feedback capacitor and a switching element. In this detector, however, it is difficult to set an initial capacitance difference between the sensor capacitor and the reference capacitor to zero. If the initial capacitance difference exceeds a capacitance of the feedback capacitor in the switched capacitor circuit, the feedback capacitor is saturated and the output of the operational amplifier becomes constant, making the detector circuit inoperable.
JP-A-2001-249028 proposes a detecting circuit having an additional circuit for removing the adverse effects of the initial capacitance (or an offset capacitance). The output of the switched capacitor circuit is converted into stored electric charges in the additional circuit. The capacitance changes in the variable capacitor are detected based on a voltage converted from the stored charges. In this manner, the adverse effects of the offset capacitance are removed. However, it is necessary to provide the additional circuit in this detector, and accordingly the detector becomes complex and expensive.